1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium and, more particularly, to a perpendicular magnetic recording medium that contains Ge oxides.
2. Background of the Related Art
Conventionally, a perpendicular magnetic recording scheme for realizing perpendicular magnetization of a surface of a magnetic recording layer (magnetization film) of a magnetic recording medium has been used as a recording scheme of magnetic recording apparatuses. A magnetic recording medium used in the perpendicular magnetic recording (hereinafter perpendicular magnetic recording medium) includes a non-magnetic substrate, a magnetic recording layer formed from a magnetic material, and the like.
In order to improve recording density of a perpendicular magnetic recording medium, the properties thereof have been changed and improved continuously. A main change or improvement is a gradual reduction in the size of magnetic crystal grains that form the magnetic recording layer. As a result, the size of state-of-the-art magnetic crystal grains approaches the physical limit called a superparamagnetic limit in which magnetization cannot be maintained stably due to the influence of ambient heat.
Thermal stability of perpendicular magnetic recording media can be improved by a magnetic recording layer formed from a material having strong magnetic anisotropy. A thermal stability index of perpendicular magnetic recording media is represented by KuV/kbT where Ku is a magnetic anisotropy constant, V is volume of a magnetic crystal grain, kb is the Boltzmann constant, and T is an absolute temperature. It is estimated that KuV/KbT needs to be 60 or more in order for recorded information to be maintained for ten years. As a method for solving the superparamagnetic limit problem, it has been deemed appropriate to use a material having a high magnetic anisotropy constant Ku.
Further, as a scheme for increasing the density of a magnetic recording layer, a method of using a granular structure in which a magnetic crystal grain is surrounded by a non-magnetic crystal grain boundary, such as an oxide or a nitride, has been proposed.
Japanese Patent Application Publication No. 2010-135610 (Patent Literature 1) discloses a structure in which a non-magnetic material is interposed between crystal grains of an alloy to form a granular film as a magnetic thin film. Examples of the non-magnetic material used for forming the granular film include SiO2, Cr2O3, ZrO2, and Al2O3. These non-magnetic materials are highly likely to magnetically separate crystal grains of Co—Pt—C-based alloys.
Japanese Patent Application Publication No. 2010-34182 (Patent Literature 2) discloses a magnetic thin film having a granular structure. This granular structure includes a ferromagnetic crystal grain mainly composed of a Co-M-Pt alloy (where M represents one or a plurality of metal elements other than Co and Pt) having an ordered structure of atoms called a L11-type and a non-magnetic grain boundary surrounding the magnetic crystal grain. The alloy is an ordered alloy based on the three elements Co, Fe, and Pt having a granular structure.
In a magnetic recording layer having such a granular structure, the non-magnetic crystal grain boundary physically separates magnetic crystal grains to reduce magnetic interaction between the magnetic crystal grains. Reduction in magnetic interaction suppresses formation of a zigzag magnetic wall occurring in a transition region of recording units to realize low-noise properties.
When a magnetic recording layer having a granular structure is formed using a conventional non-magnetic non-metallic material, the magnetic anisotropy constant and coercivity of the perpendicular magnetic recording medium are not satisfactory. For example, conventionally, materials such as SiO2 and TiO2 have been used for granularizing an ordered alloy of FePt or the like. However, these materials could not sufficiently create an ordered structure of the alloy and separate grains, and the magnetic anisotropy and coercivity thereof were decreased. Thus, a novel perpendicular magnetic recording medium having particularly high magnetic properties continues to be desired.
Moreover, a perpendicular magnetic recording layer in which an FePt-based ordered alloy is granularized has various problems. For example, the coercivity is high and the temperature at which heat-assisted recording is performed increases, and a magnetic reversal field gradient decreases when data is recorded at high temperature.
The present invention has been made in view of the above-described challenges and an object thereof is to provide a perpendicular magnetic recording medium in which magnetic crystal grains are sufficiently ordered and separated, and which has a high magnetic anisotropy constant.
Another object of the present invention is to provide a perpendicular magnetic recording medium having high coercivity in a room-temperature region in which recorded data is preserved.
Another object of the present invention is to provide a perpendicular magnetic recording medium capable of decreasing a magnetic reversal field in a high-temperature region in which data is recorded.
Another object of the present invention is to provide a perpendicular magnetic recording medium capable of increasing a temperature gradient of a magnetic reversal field in a high-temperature region in which data is recorded.